Hydrogen iodide (HI) in general chemistry
Hydrogen iodide is the covalent hydrogen halide with formula HI. The same chemical formula is named differently depending on whether it is a molecular substance (gas) or an aqueous binary acid (dissolved in water).
Step 1: Write the correct chemical formula
Hydrogen iodide contains hydrogen (H) and iodine (I) in a 1:1 ratio, so the molecular formula is HI.
Step 2: Apply naming rules (gas vs aqueous)
| Context | Formula notation | Correct name | Reason |
|---|---|---|---|
| Molecular substance | \(\mathrm{HI(g)}\) | Hydrogen iodide | Named like a binary covalent compound (hydrogen + iodide) |
| Dissolved in water | \(\mathrm{HI(aq)}\) | Hydroiodic acid | Binary acid naming: hydro- + (iod) + -ic acid |
Step 3: Describe acid behavior of hydroiodic acid
In water, hydrogen iodide acts as an acid by donating a proton to water. The dominant ionic products are hydronium and iodide:
\[ \mathrm{HI(aq)} + \mathrm{H_2O(l)} \rightarrow \mathrm{H_3O^+(aq)} + \mathrm{I^-(aq)} \]
Hydroiodic acid is classified as a strong acid in typical general-chemistry treatments, meaning ionization in water is essentially complete compared with weak acids.
Step 4: Calculate the molar mass of hydrogen iodide
The molar mass of HI is the sum of atomic masses for one H and one I. Using common periodic-table values: \(M(\mathrm{H}) \approx 1.008\ \text{g/mol}\) and \(M(\mathrm{I}) \approx 126.90\ \text{g/mol}\).
| Element | Count in HI | Atomic mass (g/mol) | Contribution (g/mol) |
|---|---|---|---|
| H | 1 | 1.008 | \(1 \times 1.008 = 1.008\) |
| I | 1 | 126.90 | \(1 \times 126.90 = 126.90\) |
\[ M(\mathrm{HI}) = 1.008 + 126.90 = 127.908\ \text{g/mol} \approx 127.91\ \text{g/mol} \]
Summary: HI is hydrogen iodide (gas), HI(aq) is hydroiodic acid, and \(M(\mathrm{HI}) \approx 127.91\ \text{g/mol}\).